lambda sensor. 850 2.5

[danshan]

Hi all, seasons greeetings!

Went up to Norfolk for Xmas and whilst out and about noticed the fuel consumption dropped considerably, down to an average 10mpg!! And this was doing 50-60 on single lane carriageways!! Got the car home to find the exhaust pipe black. I took it back to the specialist volvo dealers where i bought it.(luckily warranty doesent run out till Mid January!!). They checked it over, emissions were off the scale!! They decided it was the lambda sensor So they have disconnected it since they havent got one in stock untill begining of jan.The mpgs have improved but nothing like before.
On the trip back doing a careful 65-70mph it averaged 20mpg.

My worry is, will this have any long lasting effect. As i have already done 200miles on it with the sensor disconnected, The garage seem to think it wont but i am dubious.

Any comments appreciated.

Cheers Mark

ps. would the lambda sensor going cause the engine to run cooler, as the termp gauge has been reading only 1/4 compared to its usual 1/2 way.
by the way its a 96 glt 2.5 20v.

[sholtby]

It wants sorting asap or the cat will overheat and need replacing.

Simon

[ShadeTek]

I think your initial problem is your thermostat is faulty. I had this recently, it caused increased fuel consumption and mis-firing. You'll find the temp comes up to normal temp in slow traffic and then drop when you get moving above 40MPH(If it is your t-stat). The ECU increases fuel feed to the engine to try and get it to correct operating temperature and in doing so foul the plugs up due to an over-rich mixture. I replaced my thermostat and what a difference! It must have been needed for quite a while as consumption dropped and I got a much better heat out of the core. Your temp gauge should sit at 3 O'clock or you have a problem. Pull some codes from the ECU to see if you can narrow down the fault if the thermostat or even temp sensor is fading.

[cootuk]

How long do you think you could run rich like this without permanently damaging the cat?...my thermostats being replaced at the next service in a couple of weeks, but probably been duff for a couple of months.

Sounds like your thermostat is probably gone. Stuck open by the sounds of things. This will cause your ECU to tell the engine to burn more fuel. I don't know if this is linked to the Lambda sensor issue but I wouldn't drive it until it's replaced. Just in case.

[volvo2]

My 1995, 850, 2.5, 10V seems also to run at the 4 o'clock position, unless on the motorway when it rises to 3 o'clock. Fuel consumption averages about 34MPG. I think the thermostat must be working, at least to some extent, as it is possible to see the temperature gauge rise and then abruptly fall as the engine warms up from cold.

[danshan]

Thanks for all the advice , i have spoken to the garage and they will be replacing the thermostat aswell on friday. So hopefully that will also sort out the fuel consumption to something close to what alot of people on the forum are gettting.

Cheers again.Mark.

96 850 2.5 20V